Nucleotide sequences have been determined in two defective variants of SV40 that contain covalently linked monkey and viral DNA sequences. In each variant, part of the monkey sequences are derived from the highly reiterated portion of the cellular genome called alpha-component. However each variant contains a different member of the set of closely related, 172 base pair long monomer units contained in alpha-component. Each variant also contains sequences tentatively identified as low reiteration frequency monkey sequences: the sequences in the two variants are different. Sequences around the joints between the SV40 and monkey DNA have been determined. These studies afford information on the nature of the recombinational events involved in the evolution of the variants.